The present invention generally relates to apparatus and methods for sealing a high temperature gas and, more specifically, to apparatus and methods for sealing between the inner surface of an exhaust gas duct and a heat exchanger disposed within the duct.
A sealing arrangement for such an application must accommodate relative movement of the heat exchanger relative to the duct due to thermal expansion. This relative movement generally includes both translation and rotation.
Seals for such high temperature applications differ from the familiar gaskets, O-rings, etc employed in fluid systems at low temperatures because elastomeric materials used at low temperatures are unsuitable for service at the high temperatures encountered in applications involving hot combustion products. Seals for high temperature applications generally employ metals suitable for high temperature application, and in some cases employ refractory fibers.
An early high temperature seal is described in U.S. Pat. No. 1,073,916 which is intended for use in steam turbines to seal between two relatively displaceable parts. The seal of this invention has a connecting member having a bead on each edge. The beads fit into recesses in the two relatively displaceable parts. The edges of the recesses are bent around the beads to capture them. Assembly and replacement of the seals is therefore difficult. Furthermore, this sealing arrangement provides for a very small number of degrees of freedom of relative displacement of the two parts.
U.S. Pat. No. 3,698,743 teaches a combustion liner joint having a ring which seals between relatively moveable inner and outer sections of the combustion liner. Only one degree of freedom is cited for the relative displacement of the two sections. Furthermore, at reduced temperatures the two sections are only loosely connected.
U.S. Pat. No. 4,379,560 teaches an annular seal for sealing between two concentric tubular elements in a gas turbine. The seal is formed as an annular sheet of metal having a bead at its inner radius and a bead at its outer radius. The outer bead fits in an inwardly facing annular groove in the outer of the two concentric tubular elements and the inner bead fits in an outwardly facing groove in the inner of the two concentric tubular elements. This seal accommodates axial and radial displacement of the inner element relative to the outer element. It appears to be difficult to assemble and accommodates only a relatively small number of relative displacements.
U.S. Pat. No. 6,164,656 teaches a seal spring for sealing between relatively moving parts in a gas turbine engine. This seal has the undesirable feature that the seal spring is under stress, a fact that severely limits its life.
In short, prior art seals for high temperature application generally accommodate a small number of degrees of freedom in the relative displacement of the parts between which sealing is provided. Prior art seals are generally difficult to assemble, and in some cases include components under stress which limits their life.
As can be seen, there is a need for an improved sealing arrangement and method which facilitates assembly, accommodates a greater number of relative degrees of freedom of the relative displacement of the components between which sealing is provided, and which is not under stress when it is in use.